Cyclohexanol to Cyclohexene

Synthesis and Reactions of Alkenes: Cyclohexene from Cyclohexanol from K. L. Williamson, Macroscale and Microscale Organic Experiments, 2nd Ed. 1994, Houghton Mifflin, Boston d. p268; revised 10/10/98, adapted from the microscale synthesis posted by: http://courses.chem.psu.edu/chem35/HTML/Experiments/Exp72.pdf Prelab Exercise: Prepare a detailed flow sheet for the preparation of cyclohexene, indicating at each step which layer contains the desired product.

Introduction Dehydration of cyclohexanol to cyclohexene can be accomplished by pyrolysis of the cyclic secondary alcohol with an acid catalyst at a moderate temperature or by distillation over alumina or silica gel.

The procedure selected for this experiment involves catalysis by sulfuric acid. When a mixture of cyclohexanol and sulfuric acid is heated in a flask equipped with a fractionating column, the formation of water is soon evident. On further heating, the water and the cyclohexene formed distill together by the principle of steam distillation, and any high-boiling cyclohexanol that may volatilize is returned to the flask. However, after dehydration is complete and the bulk of the product has distilled, the column remains saturated with water-cyclohexene that merely refluxes and does not distill. Hence, for recovery of otherwise lost reaction product, a “chaser” solvent is added, and distillation is continued. A suitable chaser solvent is the water-immiscible, aromatic solvent toluene, boiling point 110°C; as it steamdistills, it carries over the more volatile cyclohexene. When the total water-insoluble layer is separated, dried, and redistilled through the dried column, the chaser again drives the cyclohexene from the column; the difference in boiling points is such that a sharp separation is possible. The column holdup (retention of distillate) in the metal sponge-packed column is so great that if a chaser solvent is not used in the procedure, the yield will be much lower. The mechanism of this reaction involves initial rapid protonation of the hydroxyl group by the sulfuric acid. This is followed by

loss of water to give the unstable secondary carbocation, which quickly loses a proton to water or the conjugate acid to give the alkene.

¥ Procedure: Preparation of Cyclohexene • Add 1.5 mL of water into a 25 mL round bottomed flask. To this, add 2.0 mL of concentrated sulfuric acid, followed by 10.0 g of cyclohexanol (note, this is 10.0 g not 10.0 mL), and a boiling chip. Put a cork on the top of the flask, and shake gently to mix the layers. Put your hand around the flask to feel the evolution of heat. Set up the arrangement for fractional distillation as performed previously. Note that the bulb of the thermometer must be completely below the side arm of the distilling head. Wrap the fractionating column and distilling head with glass wool. Be sure to thoroughly insulate the apparatus to help speed up the distillation process. (See the experiment on fractional distillation in for details of this technique.) • Distill the mixture gently on the heating mantle, collecting the distillate in a graduated cylinder. Distill until the liquid remaining in the distillation flask has a volume of about 2.0 mL and very little material is distilling; Record the boiling temperature range. The reaction mixture will turn black and a small amount of black solid will start to form due to side reactions caused by the conc. H2SO4, but this is normal. Let the assembly cool a little after removing it from the sand, remove the thermometer briefly, and add 4 mL of toluene (the chaser solvent) into the flask. Note the amount of the organic (upper) layer in the boiling flask and distill again until the volume of this layer has been reduced by about half. Transfer the contents of the vial into a 50 mL Erlenmeyer flask, and rinse the round bottom flask with a little toluene. Use toluene for rinsing in subsequent operations. Wash the toluene/product distillate with an equal volume of saturated sodium chloride solution, remove the aqueous layer, and then add sufficient anhydrous sodium sulfate to the reaction tube so that it does not clump together. Shake the solution with the drying agent, and let it dry for at least 5 min. While this is taking place, clean the distilling apparatus first with water, then ethanol, and finally a little acetone. It is absolutely essential that the apparatus be completely dry; otherwise, the product will be contaminated with whatever solvent is left in the apparatus. Transfer the dry cyclohexene solution to the distilling flask, add a boiling chip and distill the product. At the moment the temperature starts to rise above the plateau at which the product distills (83°C), stop the distillation to avoid contamination of the cyclohexene with toluene. A typical yield of this volatile alkene is about 3.5 g. Report your yield in grams and your percentage yield.

Analyses: Analyze your sample with IR and determine how differences in b.p. are related to % yield. Post Lab Questions 1. Assign the peaks of the 'H NMR spectrum of cyclohexene below to specific groups of protons on the molecule.

2. What product(s) can be obtained by the dehydration of (a) 2-heptanol (b) 2-methyl-1-hexanol? 3. Write a detailed mechanism for the dehydration of cyclohexanol to cyclohexene.